A standard nozzle assembly for making a synthetic-resin tube has an outer housing sleeve having an inner surface centered on an axis and having an axially forwardly directed front end. A core within the sleeve forms with the inner housing surface an annular axially forwardly open extrusion passage. An extruder feeds a mass to be extruded under pressure to the rear end of this extrusion passage so that it emerges either continuously or discontinuously from the front passage end as a tube.
As described in German patent document No. 1,753,883 filed Mar. 18, 1968 by H. Feuerherm, this core is held in place by short radial struts that extend between it and the outer housing sleeve. Thus the annular axially forward flow of the mass being extruded is interrupted by these struts.
Such division of the flow is usually visible as lines or striations in the finished tube. Bubbles can form where the flow reunites downstream of the support struts, weakening and even forming holes in the product which is almost invariably supposed to be gas- and liquid-impervious. As the molecular weight of the resin being extruded increases, the problems similarly increase.